THE VISCO-ELASTIC PROPERTIES OF THE LUNGS

¹ Department of Physiology, University of Edinburgh

1. The pressure required to distend the lungs under any given conditions may be divided into two parts: (a) a pressure which is positive during both inspiration and expiration (the elastic pressure), and (b) a pressure which is positive in inspiration and negative in expiration (the viscous pressure). The presence of the latter results in work being done during each complete cycle of inspiration and expiration.

2. A method is described whereby these two pressures may be independently evaluated under certain conditions. A special pump has been designed for this purpose and is described.

3. The elastic and viscous pressures developed per unit volume of tidal air are defined as the "elastance" and "viscance" respectively.

4. The viscance may be further divided into (a) that part which is due to the flow of the air through the air passages, and (b) that part which is due to the deformation of the lung structures. These may be independently evaluated by ventilating the lungs with an O₂-H₂ mixture instead of air.

5. Of the total pressure required to distend the lungs at a frequency of 18 respirations per minute, the elastance contributes 80 per cent., the structure viscance 15 per cent., and the air viscance 5 per cent.

6. The structure viscance is more or less independent of the frequency of respiration over the range of frequencies used: the phase angle of the lung substance as a visco-elastic system is thus sensibly independent of the frequency. The air viscance increases in proportion to the frequency, as is to be expected theoretically.

7. A preliminary study has been made of the action of vagal stimulation and of the administration of pilocarpine, histamine, adrenaline, and acetylcholine. In nearly every case there was a simultaneous action on both elastance and viscance, but histamine and adrenaline, on occasion, increased the elastance without having any action on the viscance, and pilocarpine and adrenaline, on occasion, increased the viscance without having any action on the elastance.

8. No evidence could be obtained that pilocarpine had any effect on the air viscance, its action being exerted mainly, or entirely, on the elastance and the structure viscance.